1. Technical Field
The present invention relates generally to a mobile communication system and, in particular, to a method for generating quasi-orthogonal codes and spreader using the same in a mobile communication system.
2. Description of the Related Art
In general, a CDMA (Code Division Multiple Access) system separates the channels by using orthogonal codes in order to increase channel capacity. For example, a forward link specified by the IS-95/IS-95A standard separates the channels by using the orthogonal codes. This channel separation method can also be applied to an IS-95/IS-95A reverse link through time alignment.
FIG. 1 illustrates the IS-95/IS-95A forward link in which the channels are separated by orthogonal codes. Referring to FIG. 1, the channels are separated by allocated orthogonal codes Wi (where i=0-63), respectively, which typically are Walsh codes. The IS-95/IS-95A forward link uses convolutional codes with a code rate R=1/2, employs a BPSK (Bi-Phase Shift Keying) modulation, and has a bandwidth of 1.2288 MHz. Accordingly, the number of available channels is 1.2288 MHz/(9.6 KHz*2)=64 (i.e., the IS-95/IS-95A forward link can separate 64 channels by using the orthogonal codes).
By selecting a modulation method and detecting the minimum data rate, the number of available orthogonal codes can be determined. However, designers of CDMA system(s) continuously strive to provide an increase in the number of the channels in order to improve the capability. However, even when a CDMA system uses the increased number of channels, the number of the available orthogonal codes are limited. In particular, increasing the channel capacity is restricted due to the limited number of the available orthogonal codes. In a mobile communication system using a variable data rate, the length of the Walsh codes depends upon the variable data rate. Thus, it is desirable to generate quasi-orthogonal codes allowing the minimum interference with the length of the Walsh codes.
It is therefore an object of the present invention to provide a device and method for generating quasi-orthogonal codes in a mobile communication system using orthogonal codes so as to increase the channel capacity, and provide minimum interference with the orthogonal codes.
It is another object of the present invention to provide a device and method for spreading a signal by using Walsh codes and quasi-orthogonal codes in a CDMA mobile communication system.
It is a further object of the present invention to provide a device and method for generating quasi-orthogonal codes which allow the minimum interference with Walsh codes of varied lengths due to the variable data rate in a mobile communication system using both the Walsh codes and the quasi-orthogonal codes.
According to one aspect of the present invention, a channel transmission device for a CDMA mobile communication system includes a first spreader for spreading at least one input signal with quasi-orthogonal codes, a second spreader for spreading another input signal with Walsh codes, and a PN (Pseudo-Noise) spreader for complex-spreading output signals of the first and second spreaders with PN sequences. The quasi-orthogonal codes are characterized in that a partial correlation value with the Walsh codes does not exceed a lowest partial correlation limit value.
According to another aspect of the present invention, a method for generating quasi-orthogonal codes of length 22m in a mobile communication system using Walsh codes and the quasi-orthogonal codes includes the steps of generating an m-sequence of length 22m and selecting sub-sequences having a period of 22mxe2x88x921 by selecting elements at intervals of 2m+1; generating non-zero sub-sequences out of the selected sub-sequences; generating 2mxe2x88x921 sequences by connecting the sub-sequences, and column-permuting the generated sequences by a column permutation function; adding Walsh codes to the column-permuted sequences to generate quasi-orthogonal candidate sequences having a full correlation value between the Walsh codes and other quasi-orthogonal codes that is smaller than a lowest full correlation limit value; and selecting, from the quasi-orthogonal candidate sequences, quasi-orthogonal codes having a partial correlation value with the Walsh codes that satisfies a minimum partial correlation value at a variable data rate.